(A)–(E) PhuZ201 subunit is colored by the following scheme: brown, N-terminal domain; yellow, H7; violet, activation domain; rosy brown, H11; cyan, C-terminal tail; GDP:Mg2+ is colored by element. Cryo-EM map density is represented as a gray mesh (D) and (E). (A) Exterior and (B) luminal views of a subunit within the filament. (C)–(E) Lateral contacts within the three-stranded filament. Alpha carbons of the residues predicted to mediate lateral contacts are shown as spheres. (C) C-terminus of a subunit #0 makes contacts with the longitudinal subunit #1 and with the subunits #2, and #3 of a lateral dimer. Filament ends are designated as (+) end with the C-termini and (−) end with the activation domains. (D) D303 and D305 of #0 contact K238 and R217 of #2 respectively; N299 of #0 contacts Q157 of #3; (E) R290 of #0 contacts E225 of #3. See also Figure S2.